The size of a someone's penis does not determine whether or not they're a good lover — all it does is determine what size condom they should pick up at the pharmacy. However, some people do have penis size preferences, with plenty of people preferring larger penises and others who really only want to get down with someone who has a smaller penis. Vaginas, just like penises, are different sizes. So a person might have a particularly short vagina, in which case having sex with someone who has a really big penis is going to be a major no. Sorry folks — that middle school thing about shoe size is a total myth.